I don't know if it is still relevant - but take a look at this thread ...
ViewMail not working on all accounts
In your admin tool - under Domains -> yourDomain -> Public service host name
what do you have ?
Is the name here DNS resolvable ?
If you copy/paste that entry into a browser - does it open when prefixed with https:// ?
If you click "Check MX record" on the toolbar - does it pass ?
Also - check out this thread - and read through the comments in the bug report. They might help you to isolate the issue.
Admin Console gives error on View Mail (5.0.9)